Transistors are semiconductor devices, commonly used as an amplifier or an electrically controlled switch. The transistor is the fundamental building block of the circuitry in computers, cellular phones, and all other modern electronic devices. Because of its fast response and accuracy, the transistor is used in a wide variety of digital and analog functions, including amplification, switching, voltage regulation, signal modulation, and oscillators. Transistors may be packaged individually or as part of an integrated circuit.
Thin film transistors (TFT) are a special kind of field effect transistor made by depositing thin films of a semiconductor active layer as well as the dielectric layer and metallic contacts over a supporting substrate. A common substrate is glass, because one of the primary application of TFTs is in liquid crystal displays (LCDs). This differs from the conventional transistor where the semiconductor material typically is the substrate, such as a silicon wafer.
TFTs can be made using a wide variety of semiconductor materials. A common material is silicon. The characteristics of a silicon-based TFT depend on the crystalline state of the silicon. That is, the semiconductor layer can be either amorphous silicon, microcrystalline silicon, or it can be polysilicon or single crystalline silicon. Other materials which have been used as semiconductors in TFTs include compound semiconductors such as cadmium selenium (CdSe) and metal oxides such as zinc oxide. TFTs have also been made using organic materials (referred to as an organic TFT or OTFT).
There has been a growing trend to fabricate TFTs on flexible substrates, such as, for example, plastics and poly films. Such substrates are lower in cost than glass and provide a wider range of applications, such as, for example, flexible displays, packaging, signage, labeling, and other similar applications. However, many difficulties arise in the manufacture of TFTs on flexible substrates, particularly plastics, particularly due to the high temperatures used in manufacturing TFTs.
Because a flexible substrate such as plastic or poly film cannot withstand the high annealing temperature, deposition, patterning, doping and other processes used in TFT fabrication must be completed under relatively low temperatures. Chemical vapor deposition, and/or physical vapor deposition (usually sputtering) are techniques often applied in the fabrication of TFTs. However, desirable performance characteristics, such as high carrier mobilities, low leakage currents and threshold voltages, for high-performance applications such as, for example, for use in liquid crystal displays, are difficult to achieve when low processing temperatures are used. Processing temperatures (<150° C.) below those used for TFT fabrication on glass substrates must be maintained for compatibility with low-cost plastic substrate materials. In general, superior TFT performance is achieved with higher-temperature fabrication processes, because crystalline material can be deposited at the higher temperatures, dopants can be activated at higher temperatures, and the quality of the critical gate-dielectric interface, which is highly sensitive to process temperature, can be controlled.
To attempt to address these and other issues, researchers in the field have tried a number of different approaches. Laser annealing of silicon has been studied for many years as a means for improving the crystalline nature of a deposited film. Typically, an amorphous silicon film is deposited and a short pulse duration excimer laser is used to melt a portion of the silicon film. As the silicon cools between pulses, crystallization may occur resulting in crystal grains of various sizes. This approach has been used with success on both glass and plastic substrates.
Laser-induced doping of silicon is a promising method that has been used to create conductive silicon film for use in thin film transistors. One implementation of laser-induced doping is known as gas immersion laser doping (GILD). In a GILD process, a laser pulse induces the melting of silicon in the presence of a precursor, or dopant, gas. Part of the gas species already chemisorbed or impinging on the melted silicon surfaces diffuses into the molten silicon. As a result of the melting and solidification cycle in the presence of the dopant gas, the dopant is incorporated into the silicon layer. Using this approach and/or similar approaches, researchers have been able to achieve high dopant concentrations (both N-type and P-type) and excellent spatial dopant profiles, all while retaining highly crystalline silicon and low surface roughness. Details of this process can be found in: G. Kerrien et al., App. Surf. Sc. 186 (2002) at 45-51, and A. Slaoui et al., J. Appl. Phys 67 (10) 1990, p. 6197.
Even though advances have been made in creating polycrystalline thin films, doping silicon films at low temperature and fabricating TFTs using these thin films, there remains a need for relatively low-cost systems and simplified methods for fabricating a high-performance Si-based TFT on a flexible, plastic substrate.